Loudspeaker



April 27, 1937. A. A. THOMAS 2,078,469

LOUDSPEAKER Original Filed Sept. 15, 1928 2 Sheets-Sheet 1 lNVENT OR 1928 2 Sheets-Sheet 2 April 27, 1937. A. A. THOMAS LOUDSPEAKER Original Filed Sept. 15,

INVENTOR m ill-5E Patented Apr. 27, 1937 UNITED STATES PATENT OFFICE LOUDSPEAKER a corporation of Delaware Application September 15, 1928, Serial No. 306,172..

Renewed November 20, 1935 24 Claims.

This invention relates to the art of loudspeakers of the type in which a movable speech coil operates in a magnetic airgap. One feature of my new driving unit comprises a'right-angled lever pivoted at its apex for transmitting the vibrations of the coil to a loudspeaking diaphragm in predetermined ratio. The coil and the diaphragm may be mounted on the two ends of the right angled lever or bellcrank, which thus serves as a common support for those two parts. In one form of my invention, the speech coil is adjustably mounted on the bellcrank to vary its normal position in the magnetic field, whereby the quality of reproduction may be controlled within certain limits. The bellcrank may be mounted on a resilient pivot, such as a laterally twistable spring blade supported at its ends. The speech coil may be cylindrical to vibrate in an annular airgap, or it may be of flat rectangular shape arranged to vibrate at opposite sides in a pair of aligned narrow airgaps. To neutralize (or at least minimize) the disturbing inductive effects of a cylindrical speech coil on the field magnet, I provide the latter with a compensating coil connected in the speech circuit in opposing magnetic relation to the speech coil and surrounding the same. Considered as a whole, my new electrodynamic driving unit is characterized by simplicity of structure, ease of assembly, and

efiiciency of operation.

The various features and advantages of my invention will be fully understood from a detailed description of the accompanying drawings, in which- Fig. 1 is a sectional view of a dynamic cone speaker constructed in accordance with my invention;

Fig. 2 is a detached perspective view of the resilient mounting for the bellcrank that connects the speech coil to the diaphragm;

Figs. 3 and 4 are respectively side and edge views of a sheet metal bellcrank to which the speech coil is rigidly connected;

Fig. 5 illustrates a modification employing a fiat speech coil, this view being partly in section;

Fig. 6 is a plan view of Fig. 5, except that the diaphragm has been omitted for lack of space;

Fig. 7 is an end view of Fig. 5, partly sectioned on line 'l-l of Fig. 5; I

Fig. 8 is a section on line 8--8 of Fig. 7; and

Fig. 9 is a fragmentary sectional view on line il--9 of Fig. 5, showing the pivot support for the bellcrank.

Referring to Figs. 1 and 2, there is a casting indicated as a whole by liLkwhich is mounted on a suitable base l2 by screws l3, or otherwise. The casting it], which may be of brass, aluminum, or other suitable material, comprises a base plate i4 and a pair of integral arms or posts l5. If 5 the base plate I4 is made of sheet metal, the arms l 5 may also be of sheet metal and attached to the base plate as separate pieces. The arms or posts l5 terminate in lateral extensions Hi, to which a bar or cross-plate I7 is rigidly connected 10 by suitable fastening members l8, such as bolts, screws, rivets and the like. The bar I! is preferably a casting of aluminum, brass, or like material. As best shown in Fig. 2, the cross-bar H has a pair or lugs I?) provided with slots 20 for receiving the ends of a spring blade 2|. To place the spring blade 2| under initial tension, the slots 2E3 slantin opposite directions, so that the two halves of the spring are slightly twisted and thereby tensioned in opposed directions. C'on- 20 sequently, the center of the spring is always in a state of balanced or neutralized tension which opposes movement of the spring in either direction. In other words, the center of the oppositely twisted spring blade 2| becomes in effect 25 a resilient pivot which automatically assumes a predetermined normal position. The ends of spring blade 2| are secured in the slotted supporting blocks or lugs l9 by means of screws, bolts or rivets, 22, or' in any other practical way. 30 The members H! are preferably integral with bar it, but they may be secured theretoas separate pieces.

A bellcrank or right-angled lever 23 is mounted at its apex on the center of the spring blade 2|. 35 A simple way to connect these two parts is to provide the bellcrank at its apex with a slot 24, through which the spring blade 2| passes in a tight frictional fit. If necessary or desirable, separate means may be used to prevent lateral 40 movement, of the bellcrank on the supporting spring, such as a pair of small pins passing through the spring blade and engaging opposite sides of the bellcrank. In Figs. 1 and 2 I have assumed that the bellcrank 23 is a single casting 45 (e. g., aluminum). The horizontal arm 25 of the bellcrank is provided with an integral hub or bearing 26, and the vertical arm 21 terminates in a perforated lug 28.

A loudspeaker diaphragm 29, which is here 50 shown in the form of a conventional cone, carries at its center a tubular extension 30, which has a conical enlargement 3| at one end to facilitate attachment of the diaphragm by fastening members 32 The tubular connecting piece 30 is screwthreaded inside to receive a screw 33, which passes through the perforated lug 28 of bellcrank 23, so that the diaphragm 29 is rigidly secured to the upper end of the pivoted bellcrank. A set-screw 34 may be used to prevent possible loosening of the screw 33 due to the vibration of the parts. Since the diaphragm is directly supported on the bellcrank, there is no need of connecting the diaphragm at its periphery to a separate support, although that may be done if necessary. Some manufacturers prefer to use a freeedged cone, and this can be done here, because the diaphragm 29 can be left free at the edge to vibrate bodily with substantially uniform amplitude over its entire surface. The diaphragm 29 may be constructed of stiff paper, sheet metal, celluloid, stiffened fabric and other materials suitable for the purpose. I do not limit myself to any particular form or style of loudspeaking diaphragm.

The base plate l4 carries an electromagnet indicated as a whole by M, which comprises an outer magnetic shell 35 and an inner central core 36. These two parts are of magnetic material having a high degree of permeability. The magnet M is held in position by a screw 31, which may also be used to secure the polar core 36 to the outer shell 35. A coil 38 is mounted on the magnet core 36 to produce a substantially constant field of the requisite strength. The upper end 39 of shell 35 is of slightly smaller diameter than the lower portion of the shell to provide an annular shoulder 43 for supporting a coil 4!, the purpose of which will presently be explained. The polar core 33 and the upper section 39 of shell 35 form an annular airgap 42 in which a speech coil 43 is mounted for axial vibration. The coil 43 is mounted on a cylindrical support 44, which is preferably a thin shell of light non-magnetic material, such as stiff paper, aluminum, bakelite, fiber, and others along the same line. The cylinder 44 is formed with a recess 45 (see Fig. 3) for holding the coil 43, and this recess is formed between a bead 44a and a shoulder 44?) of the cylinder. The hub 26 on the free end of the horizontal arm 25 of bellcrank 23 has a screwthreaded axial opening 46 for receiving a screw 41 to which the coil support 44 is connected. By turning the screw 41, the normal position of coil 43 in the magnetic airgap 42 is correspondingly adjusted in an axial direction. This adjustment to some extent controls the quality and volume of the reproduction. A coil spring48 on screw 41 may be used to prevent loosening or rattling of the screw during the operation of the instrument.

The coil 4|, which surrounds the speech coil 43 in substantially concentric relation, is preferably connected in opposing series with the speech coil, and these two coils are so wound or connected as to produce opposing magnetic effects on the field flux of the magnetizing coil 38. It will not be necessary to show the circuit connections of the coils 38, 4| and 43, because such connections are well known and will be understood by those familiar with this art. It is enough to say that the magnetizing coil 38 is connected with a source of substantially constant current, and that the coils 4| and 43 are connected in a suitable amplifying circuit, of which many types are on the market.

Concerning the operation of the instrument, I need not go into details, because the functioning of dynamic speakers is pretty well understood. The passage of current impulses through the speech coil 43 causes the same to vibrate axially in the magnetic airgap 42. The direction and extent of this vibration depend upon the direction and intensity of the current at any moment. The bellcrank 23 transmits the movements of coil 43 without lost motion to the diaphragm 29. The tensioned spring blade 2 l, which is laterally twistable at the center to permit pivotal vibration of the bellcrank 23, also acts as a restoring spring for the bellcrank and all parts carried thereby, including the diaphragm 2E} and coil 43. The elimination of a separate restoring spring simplifies the construction. The transmission ratio between the coil 43 and diaphragm 29 depends upon the relative lengths of the arms 25 and 22". In Figs. 1 and 2, the horizontal arm 25 is longer than the vertical arm 27, so that the coil vibrations are transmitted to the diaphragm in reduced ratio, but with correspondingly increased power. In some instances it may be best to make the arms 25 and 271 of equal length, or the vertical arm 21 may be longer than the horizontal arm 25. The relative lengths of the right-angled transmitting arms can easily be determined by experiments well within the skill of the artisan.

In Figs. 3 and 4,. the bellcrank 23 is bent from a single piece of sheet metal, preferably aluminum because it is light and strong. The upper end, of the vertical arm 2? terminates in a loop 49 adapted to receive the screw 33, and the end of the horizontal arm 25 terminates in a flat extension 56 to which the coil-supporting shell 44 is rigidly attached, as by rivet 5i.

In the modification illustrated in Figs. 5-9, there is a non-magnetic supporting base 52 having an upright extension 53, which is provided with a pair of spaced lugs 54 carrying pointed bearing screws 55. The bellcrank 23 is in this instance provided at its apex with opposite recesses 56 for receiving the pointed ends of the bearing screws 55, whereby the bellcrank is pivoted at its apex to vibrate without lost motion. If desired, the twistable spring blade 2! of Figs. 1 and 2 may be used in place of the pivot screws 55, but in this case it is not necessary to have a resilient mounting for the bellcrank. The upper end of bellcrank 23 carries a screwthreaded hub or sleeve 51, which may be attached by a rivet 58 or cast integral with the bell-crank. A cone diaphragm 58 is connected to the sleeve 57 by a screw 60, which may be locked against loosening by a set-screw 6!. What has been said about the diaphragm 29 of Fig. 1 may be considered applicable to the diaphragm 59 of Fig. 5, except that in this modification the periphery of the diaphragm is clamped between a pair of rings 62 and 33by fastening members 34, which may be screws, bolts, rivets and the like. The ring 63 is rigidly supported by arms 65, which extend from the vertical plate 53 of base 52. For convenience of manufacture, the parts 52, 53, 53 and 65 may be cast as 'a unit; or, the base mem ber 52 may be a heavy casting like brass, while the parts 53, 63 and 65 are cast of aluminum or other light metal and secured to the heavy base 52, whereby the mechanical stability of the instrument is increased.

The base 52 has a pair of aligned rails or blocks 66 for supporting a, pair of electromagnets indicated as a whole by E and E. These field magnets consist each of a pair of pole pieces 67 and 68 connected by a core 69, which carries a magnetizing coil lil. Each magnet may be cast as a single U-shaped unit, but it is desirable to have at least one of the pole pieces separate to permit the use of machine-wound coils; In Fig. 7, the core 69 is integral with the lower pole piece 68 and the upper pole piece 61 is a separate member attached by a bolt H, which is preferably of magnetic metal. The coil 10 is machine-wound and slipped over the core 69 before the pole piece 61 is bolted in place. Any suitable expedient may be employed to prevent lateral movement of pole piece 67 on core 69. In Fig. '7, I have shown the core 88 provided with a rectangular recess or slot 12 adapted to receive a correspondingly shaped projection 13 on pole piece Bl. If desired, the core 59 can be made separate from both pole pieces.

It goes without saying that the parts comprising the field magnets E and E are made of magnetic material having a high degree of permeability, and they may be cast solid or composed of laminae, whichever type of magnet a manufacturer may prefer. The rails or blocks 56, which may be integral with base 52 or attached thereto as separate pieces, are provided with recesses 74 for receiving the pole pieces 68 in a snug fit. The bolts 1| have nuts 15 for holding the parts 61, 68, 69 rigidly clamped together. The underside of each pole piece 68 has a recess 15 for accommodating the nuts 15, which therefore do not interfere with the proper mounting of the magnetic frames on the blocks 66. The bolts 1| pass through slots il in blocks 66 and carry clamping nuts l'8. It will thus be seen that the bolts H perform the double function of holding the parts of the electromagnets E and E in assembled condition and supporting the electromagnets as a whole in rigid position on the rails or blocks 66. The slots ll permit slight adjustment of the magnets toward and from each other to regulate the normal width of the two aligned airgaps 19. The magnetizing coils 18 are so wound that their magnetic elTect is cumulative; that is to say, the flux passes through the airgaps 19 in series. The pole pieces 61 and 68 preferably terminate in lateral extensions 8!] to provide airgaps of increased length or depth, for a purpose that will presently be clear.

A spring blade 8| is connected at its ends to the base 52 by means of screws 82 and blocks 83. In some instances it may be desirable to maintain the spring 82 under initial longitudinal tension. A simple and efficient way to produce this initial tension is to provide the base 52 with a pair of rounded bearing lugs or ribs 84 against which the spring 8| is forced by set-screws 85 mounted in the clamping blocks 83. As seen in Fig. 5, the screws 85 are offset with respect to the lateral contact ribs 84, so that by tightening the screws 85, the central portion of the spring is stretched between the ribs. A flat rectangular frame indicated as a whole by 86 is connected at diametrically opposite ends to the bellcrank 23 .and the spring blade 8|. The frame 86 is preferably a light rectangular structure of non-magnetic sheet metal (such as brass, aluminum, etc.), and has parallel side flanges between which a speech coil 81 is supported. The flanges of frame 86 are provided with upper extensions 88 and lower extensions 89. The horizontal end of bellcrank 23 passes between the upper extensions 88, and a fastening member 98 connects coil frame 86 to the bellcrank. The connecting point 90 may be a pivot, or it may be rigid, since the movements of the coil frame are exceedingly small. The lower extensions 89 on coil frame 86 are bent laterally to overlap at their ends. A bolt or other fastening member 9| passes through the spring blade 8| and the overlapping ends of extensions 89 for rigidly connecting the coil frame to the center of the spring blade. Any other practical means may be used for establishing this connection.

The upper and lower horizontal sides of the flat rectangular coil 81 vibrate in the aligned airgaps F9 in a plane at right-angles to the passage of the flux across the airgaps. The purpose of the lateral extensions 88 on pole pieces 61 and 68 is to include as great a length of coil as possible in the magnetic field, whereby the driving power of the unit is increased. The non-magnetic coil frame 86 need only be slightly narrower than the air- .gaps is. The spring 8| and belldrank 23 prevent lateral movement of the coil frame 86, so that it can not strike the sides of the adjacent pole pieces. The normal tension of the spring blade 8| is so adjusted that it furnishes the right amount of resistance to the movements of the coil frame. The spring blade 8| of Fig. 5, like the spring blade 2| of Figs. 1 and 2, performs the additional function of automatically restoring the connected parts to normal position. If desired, the diaphragm 59 may have a radially corrugated portion 92 near the periphery to act as a spring support for the main cone-shaped body of the diaphragm.

It will be clear from the preceding description that the electrodynamic speaker of my invention is simple and compact in construction, requiring but relatively few parts which are rugged and can not get out of order. The resilient mounting or connections for the speaking coils 4S and 87 eliminate lost motion, and the vibratory unit can be made light enough to have negligible inertia, so that the instrument responds efliciently to the entire musical range. Furthermore, the right-angled lever connection between the coil and the diaphragm permits the use of a selected ratio of transmission. Thus, in Fig. 1 the coil movements are transmitted to the diaphragm in decreased ratio but with increased power, whereas in Fig. 5 the transmission ratio is the other way round. Another practical advantage of the bellcrank connection lies in the fact that the field magnet and coil can be mounted on a horizontal base in upright position. To make the drawings as simple as possible I have omitted a housing or casing that encloses the parts in the finished commercial product. It is hardly necessary to add that the drawings are not made tomathematical scale; rather have the relative dimensions and arrangement of the parts been purposely exaggerated to promote clearness in the various views.

Although I have shown certain specific constructions, I want it understood that my invention is not limited to the details set forth. It is to be expected that changes and modifications will probably occur to builders of the instrument, without departing from the scope of the invention as defined in the appended claims.

I claim as my invention:

1. In a loudspeaker, the combination of electromagnetic actuating mechanism having a vibratory element, an acoustic member in the form of a direct-acting cone arranged with its concave side toward the front, a transmission member connected at one end to said element, a short tube having interior screwthreads and arranged to the rear of said cone outside of its concave space, said transmission member being arranged outside of said tube, a screw passing through one of said members into said tube for securing one end of the latter to said member, and means for securing the other end of said tube to the other member, whereby said connecting tube is supported solely by and between said members.

2. A loudspeaker comprising a fiat coil movable in its own plane in a magnetic field, a diaphragm' arranged with its axis substantially at right angles to the direction of movement of said coil, and a right-angled lever pivoted at its apex for operatively connecting said coil and dia phragm in predetermined transmission ratio, said lever being arranged to move in the operating plane of said coil.

3. A loudspeaker comprising a fiat coil movable in its own plane in a magnetic field, a dia-- phragm arranged with its axis substantially at right angles to the direction of movement of said coil, a right-angled lever pivoted at its apex for operatively connecting said coil and diaphragm in predetermined transmission ratio, said lever being arranged to move in the operating plane of said coil, and resilient means connected to said coil for holding the same in predetermined normal position.

l. A loudspeaker comprising a flat coil movable in its own plane in a magnetic field, a diaphragm arranged with its axis substantially at right angles to the direction of movement of said coil, a rightangled lever pivoted at its apex for operatively connecting said coil and diaphragm in predetermined transmission ratio, said lever being arranged to move in the operating plane of said coil, a tensioned spring blade supported at its ends, and means for connecting one end of said coil to the center of said spring blade, whereby said coil is normally held in predetermined position in the magnetic field.

5. In a loudspeaker, the combination of a pivoted lever carrying a support, a cylindrical coil mounted on said support, means for providing an annular magnetic airgap in which said coil operates, and means for adjusting said support on said lever to adjust the coil axially in the airgap, said adjusting means being operable while the coil is in said airgap.

6. In a loudspeaker, the combination of a pivoted lever carrying a support, a cylindrical coil mounted on said support, means for providing an annular magnetic airgap in which said coil operates, and screw-threaded means for turning said support in either direction to adjust the coil axially in the airgap, said screw-threaded means being operable while the coil is in said airgap.

'7. A loudspeaker comprising a bellcrank pivoted at its apex, a coil connected to one end of said bellcrank, a field magnet providing an airgap in which said coil vibrates to cut the magnetic flux, a diaphragm connected to the other end of said bellcrank, and means for adjusting the normal position of said coil in the airgap, said adjusting means being operable While the coil is in said air ap.

8. A loudspeaker comprising a bellcrank pivoted at its apex, a cone diaphragm, an internally screwthreaded sleeve arranged between the apex of said diaphragm and one end or" said bellcrank, one end of said sleeve engaging the diaphragm and the other end engaging the bellcrank, a screw engaging said member for connecting said diaphragm to the bellcrank, said sleeve being relatively short to provide a direct rigid connection between the bellcrank and the diaphragm, a vibratory coil connected to the other end of said bellcrank, and means for providing a magnetic airgap for said coil.

9. A loudspeaker comprising a bellcrank pivoted at its apex, a cone diaphragm carrying at its center a tube screwthreaded inside, said tube being relatively short and terminating at one end in an integral enlargement which is secured to said diaphragm, a screw passing through one end of said bellcrank into screwthreaded engagement with said tube for connecting said diaphragm to the bellcrank, and vibratory electromagnetic means connected to the other end of said bellcrank for actuating the same.

10. A loudspeaker comprising a bellcrank pivoted at its apex, a cone diaphragm carrying at its center a screwthreaded projection, a screw passing through one end of said bellcrank into screwthreaded engagement with said projection for connecting said diaphragm to the bellcrank, said project-ion being shorter than either arm of said bellcrank to establish a direct rigid connection between the bellcrank and the diaphragm, a vibratory coil connected to the other end of said bellcrank, and means for providing a substantially constant magnetic field for said coil.

11. A loudspeaker comprising a bellcrank pivoted at its apex, a cone diaphragm carrying at its center a screwthreaded projection, a screw passing through one end of said bellcrank into screwthreaded engagement with said projection for connecting said diaphragm to the bellcrank, a coil mounted on the other end of said bellcrank, a magnet structure having a magnetic airgap for said coil, and means for adjusting the normal position of said coil in said airgap, said adjusting means being operable While the coil is in the airgap.

12. A loudspeaker comprising a bellcrank pivoted at its apex, a cone diaphragm carrying at its center an axial tubular projection having interior screwthreads at one end and a conical enlargement at the other end, the front side of said diaphragm. being concave and said projection being shorter than either arm of Said bellcrank, means for securing the diaphragm to said enlargement, a screw passing through one end of said bellcrank into screwthreaded engagement with said projection for connecting said diaphragm to the bellcrank, releasable means on said projection and engaging said screw to lock the latter against turning, and vibratory electromagnetic means connected to the other end of said bellcrank for actuating the same.

13. A loudspeaker comprising a tensioned spring blade supported at its ends for vibratory movement in a direction at right angles to the plane of the blade, a flat frame carrying a coil arranged to vibrate in its own plane in a magnetic field, means for connecting one end of said frame to the center of said spring blade, the plane of vibration of said coil and frame being substantially at right angles to the plane of the spring blade, a diaphragm, and a bellcrank connection between the other end of said frame and said diaphragm.

14. In a loudspeaker of the moving coil type, the combination of a movable support, a frame connected to said support and carrying a coil, a field magnet having an airgap in which said coil operates, and means for adjusting the normal position of said coil frame on said support, said adjusting means being operable while the coil is in the airgap.

15. In a loudspeaker of the moving coil type, the combination of a field magnet providing an annular airgap, a cylindrical speech coil mounted for vibratory movement in said airgap, a diaphragm operated by said speech coil, and a second cylindrical coil supported on said magnet outside the annular airgap and in concentric relation to said speech coil, said two coils being connected in series and separated by an annular portion of said field magnet.

16. In a loudspeaker of the moving coil type, the combination of a bellcrank pivoted substan- 1 tially at its apex, said bellcrank consisting of a flat sheet metal strip movable in its own plane and having one end turned substantially at right angles to the main body portion, a coil form having a flat end secured to the turned end of said bellcrank, a speech coil mounted on said form, means for providing a magnetic airgap for said coil, and a diaphragm connected to the other end of said bellcrank.

17. In an electromagnetic sound reproducer, the combination of a magnet energized by direct current and having an airgap, a dynamic coil operatively located in said airgap and adapted to be energized by current impulses representing transmitted sound, a second coil externally of the airgap serially connected to said dynamic coil, and means for magnetically shielding said second coil from at least a portion of the stray flux of said magnetic field extraneous to said airgap.

18. In an electromagnetic sound reproducer, the combination of a field magnet structure comprising a pot provided with a core, which forms an annular airgap with said pot, a magnetizing coil on said core, a dynamic coil operatively mounted in said airgap, a compensating coil mounted on said pot externally of the airgap and connected in series with said dynamic coil, and acoustic means operated by said dynamic coil.

19. In a loudspeaker, the combination of a non-magnetic base, a field magnet structure 4 mounted on said base and consisting of a pair of U-shaped magnets, each magnet consisting of an L-shaped member secured to said base, and

a straight bar removably attached to the top of said member, a magnetic coil mounted on the stem of each L-shaped member, said removable top bars permitting the use of machine-wound coils, the pole pieces of said magnet being arranged to provide two aligned airgaps in series, an armature structure movable in said airgaps,

and acoustic means operated by said armature structure.

20. In an electromagnetic sound reproducer the combination of a magnet including means for the energizing thereof by direct current, an air gap in the magnetic circuit of said magnet, a

dynamic coil adapted to be energized with sound representing current operatively located in said air gap, a second coil associated with said magnetic system connected in opposing series with said dynamic coil, and a magnetic shield substantially isolating said second coil from stray fields of said magnetic system extraneous to said gap not effective on said dynamic coil.

21. In a loudspeaker, an electromagnet comprising a cylindrical shell provided with a central pole piece arranged to form an annular airgap, a cylindrical speech coil mounted to vibrate in said airgap, a winding mounted on said shell externally of the airgap and surrounding said speech coil, said winding being connected in series with said coil, and a diaphragm operated by said coil.

22. In a loudspeaker, a cylindrical magnetic shell having a section of smaller diameter at one end, a cylindrical pole piece mounted centrally in said shell, a coil for magnetizing said shell and pole piece, the adjacent ends of said shell and pole piece forming an annular airgap, a cylindrical speech coil mounted to vibrate in said airgap, a winding mounted on said smaller section of the shell externally of the airgap and connected in series with said speech coil, and a diaphragm operated by said speech coil.

23. In a loudspeaker, a cylindrical magnetic shell open at one end and closed at the other, a

cylindrical magnetic post secured to the closed bottom of said shell in substantially axial alignment therewith, a magnetizing coil surrounding said post, the adjacent ends of said shell and post being spaced to form an annular airgap, a cylindrical speech coil mounted to vibrate in said airgap, a diaphragm operatively connected to said speech coil, and a winding mounted on said shell externally of the airgap and connected in series with said speech coil.

24. In a loudspeaker, a cylindrical magnetic shell open at one end and closed at the other, a cylindrical magnetic post secured to the closed bottom of said shell in substantially axial alignment therewith, a magnetizing coil surrounding said post, the adjacent ends of said shell and post being spaced to form an annular airgap, a cylindrical speech coil mounted to vibrate in said airgap, a diaphragm operatively connected to said speech coil, a shoulder on the outside of said shell, and a winding mounted on said shell and supported at one end by said shoulder, said winding being connected in the circuit of said coil in series therewith.

ADOLPH A. THOMAS. 

